Process for refining low-phosphorus pig iron to make steel

ABSTRACT

The invention relates to a process for refining low-phosphorus pig iron in a convertor, in which one or several tuyeres are arranged in the refractory brick-work through which oxygen surrounded by a liquid or gaseous protective agent is injected below the surface of the bath. By low-phosphorus pig iron is meant a pig iron containing in percent by weight: P 0.02 - 0.5% Mn 0.1 - 3% Si 0.

United States Patent [1 1 Kniippel et a1.

[ Dec. 16, 1975 PROCESS FOR REFINING LOW-PHOSPHORUS PIG IRON TO MAKE STEEL [75] Inventors: Helmut Kniippel; Karl Brotzmann;

Hans-Georg Fassbinder, all of Sulzbach-Rosenberg, Germany [73] Assignee: Eisenwerk-Gesellschaft Maximilianshutte mgH, Sulzbach-Rosenberg, Germany [22] Filed: Sept. 22, 1972 [21] Appl. No.: 291,434

Related US. Application Data [63] Continuation-impart of Ser. Nos. 821,802, May 5,

1969, Pat. No. 3,771,998, and Ser. No. 119,258, Jam 18, 1971, Pat. No. 3,774,894.

[52] US. Cl. 75/60; 75/51; 75/52;

75/59 [51] Int. Cl? C21C 5/34 [58] Field of Search 75/60, 51, 52, 53, 59

[56] References Cited UNITED STATES PATENTS 2,855,293 10/1958 Savard ..75/60 3,079,249 2/1963 Moustier 75/52 3,330,645 7/1967 Moustier...

3,556,773 1/1971 Grenfell 75/51 3,807,989 4/1974 Nilles 75/60 FOREIGN PATENTS OR APPLICATIONS 1,450,718 7/1966 France ..75/60 585,737 1/1960 Belgium ..75/60 Primary ExaminerPeter D. Rosenberg Attorney, Agent, or Firm-Lawrence 1. Field [57] ABSTRACT P 0.02 0.5% Mn 0.1 3% Si 0.1 2%

9 Claims, N0 Drawings PROCESS FOR REFINING LOW-PHOSPHORUS PIG IRON TO MAKE STEEL This application is a continuation-in-part of our application Ser. No. 821,802 filed May 5, 1969, and now US. Pat. No. 3,771,998 and of Ser. No. 119,258 divided from said application, and filed Jan. 18, 1971, and now US. Pat. No. 3,774,894.

Refining low-phosphorus pig iron with pure oxygen which is introduced below the surface of the bath and preferably through the bottom of a convertor leads to phenomena which are unknown when practicing oxygen top blowing methods. With high-phosphorus pig iron for which the above described process was originally developed, it is a particular advantage that the blowing behavior of the melt compared to both the well known Thomas process and to the oxygen top blowing process, is essentially better. Therefore, it was surprising that with low-phosphorus pig iron, after a short refining time a vigorous slopping appeared. A decrease of this slopping could only be attained by a considerable reduction of the amount of oxygen injected per unit of time, i.e. in general to less than half of the usual amount. A significant decrease of production results. This phenomenon is so severe with low-phosphorus pig iron that the refining of low-phosphorus pig iron with pure oxygen through the bottom of a convertor be comes extremely difficult.

Proposals have already been made to improve the blowing behavior of a bottom blown convertor oper ated with pure oxygen by feeding the lime in a finelumpy form onto the melting bath. By this measure, an improved blowing behavior is obtained, however, a slopping-free refining with short refining periods is not achieved by applying that practice.

It has also been proposed to refine high-phosphorus pig iron in a bottom blown convertor adding simultaneously lime powder to the oxygen stream, in order to obtain an early dephosphorization, as this is attained also at the oxygen top blowing process when blowing high-phosphorus pig iron with lime powder. An improvement of the blowing behavior was not primarily sought in this case, because a relatively quiet blowing behavior is obtained with highphosphorus pig iron even without application of lime powder. On the other hand, when refining low-phosphorus pig iron, an early dephosphorization plays only a minor part.

Now, it has been found that an absolutely quiet slopping-free refining with short refining times is achieved in the refining of low-phosphorus pig iron with injection of the oxygen below the surface of the bath, and preferably through the bottom of a convertor, if, at least a part of the lime and preferably the whole amount of lime, is injected together with the oxygen in the form of powdery or fine-grain lime. This improvement of the blowing behavior is so considerable that the refining time can be reduced to less than half of that time, just attainable without lime addition and substantial slopping, and that the reduction in refining time is achieved without the least occurrence of slopping. F urthermore, the improvement appears particularly important when the lime is not injected in uniform manner during the whole refining period.

Furthermore, it is in the sense of this invention to combine the measures leading to a considerable im provement of the blowing behavior, with other measures resulting in special metallurgical advantages or in a particularly economic process, e.g. by using minimum amounts of slag. It has been found, for instance, that life of the refractory lining of convertors is increased considerably, the desulphurization is improved appreciably, and the refining proceeds absolutely sloppingfree when during the oxidizing of the silicon an amount of lime is injected which corresponds to from twice up to four times the weight of the silicon to be oxidized. The loading of the oxygen with lime powder is under such conditions about 2.3 to 4.6 kg/Nm of oxygen. To obtain a most favorable desulphurization without impairing the blowing behavior, it is expedient to inject the total amount of lime that has to be blown in during the desiliconizing, with the highest possible load of lime at the beginning of the desiliconizing. Thus, it is, for instance, more advantageous instead of loading the oxygen uniformly with an average amount of lime powder of 2.5 kg/Nm of oxygen during the desiliconizing period, to inject during the first half of the desiliconizing time an amount of lime powder of 5 kg/Nm of oxygen. For such a practice, it is necessary to provide for an essentially higher capacity of the lime feeding device than would be required in case of a uniform feeding. According to the silicon content, 30 to 60% of the lime powder are injected with this practice during the oxidizing of the silicon.

The same effect upon the blowing behavior appears when a part of the lime powder is blown in before the proper refining period together with an inert gas, e.g. nitrogen or an oxygen-nitrogen mixture. This practice results simultaneously in an efficient desulphurization of the pig iron. In general, there are injected about 20% of the lime during a time which corresponds to about 10% of the refining time.

The nitrogen in the application of an inert gas for desulphurization may be replaced by other gases, for instance by argon, if low nitrogen contents in the steel are important. Furthermore it has proven advantageous to continue the flow of protective medium in the form of liquid and/or gaseous hydrocarbons through the annulus of the tuyeres during desulphurization, while the inert gas loaded with lime powder flows through the inner tube. By this measure the desulphurization can be enhanced to a certain degree.

After the high lime load at the beginning of the refining process, it is advantageous to continue refining with a low lime load which amounts to about 10% of the lime load during the initial period, or without any addition of lime. In case, some slopping appears with that practice, it can again be brought fully under control within the short time of about 10 to 20 see. by a short addition of a high amount of lime, corresponding approximately to the same loading as at the beginning of the refining process.

In order to influence the blowing behavior most favorably with minimum amounts of lime, a pulsating addition of lime is particularly expedient at which during a time of approximately 10 see, one time each minute, lime powder with a loading of about 5 kg/Nm of oxygen is added.

Sometimes it is necessary for metallurgical reasons to apply during the refining a very high lime load for a short time of about 1-3 minutes within a total refining time or approximately 10 minutes, in order to obtain an efficient dephosphorization nearly independent of the carbon content. The duration of that period where the high lime load is used, is determined by the phosphorus content of the heat. In case of a. phosphorus content of the pig iron of 0.25%, for instance, a refining with an amount of oxygen of 3 Nm lmin. per ton of steel is sufficient using a lime load of 4.5 kg/Nm of oxygen for a time of 2 minutes or a lime load of 3 kg/Nm of oxygen for a time of 3.5 minutes. Higher loads of lime result in a better dephosphorization. This practice is important for steels being produced as so-called catch-carbon heats, i.e. steels, the carbon of which is not oxidized down to a low carbon content. For catch-carbon heats, at least about 90% of the lime should be no coarser than 0.1mm.

For heats being refined down to a low carbon content, i.e. heats which are not produced as so-called catch-carbon beats, a smaller amount of lime is required to remove the phosphorus. In such cases, it is sufficient to add a total amount of lime which produces a CaOSiO ratio of about 2.5 in the slag, in order to arrive at final phosphorus contents as low as about 0.01%. It can be expedient there to add the remaining amount of lime with high load at the end of the blowing process, i.e. about 10% of the total refining period. By this procedure the foaming slag which appears sometimes at the end of the refining process and which in general is difficult to govern, is completely avoided.

By the practice of this invention, as herein described it becomes possible to refine steel without any slopping in the extremely short total refining time of about 10 minutes in a convertor having the relatively small specific volume of about 0.5-0.9m per ton of steel. All former measures have only brought limited improvements, while the teaching of the present invention gives the possibility to achieve a so far unknown completely quiet blowing behavior under economically extremely favorable conditions. Furthermore, it has come out, that the rapid refining obtained by the slopping-free mode of operation together with the low specific convertor volume leads to a strong rotational motion of the bath permitting a quick solution of the added scrap.

Carrying through the process according to the invention, it was observed that coarse dust having a grain size of about 0.1 to 1mm is discharged from the convertor. This coarse dust consists of about 40% lime, 3040% iron and iron oxide, l20% SiO and a remainder of impurities, e.g. MnO. In general, the usual dust removing plants can separate this coarse dust without difficulties. Nevertheless, it is desireable to take measures to prevent the formation of such dust. The following measure has proved itself particularly efficient in this case: instead of a part of the scrap, ore is added as coolant. Preferably the addition of ore begins after about of the total refining time, and it is added in portions or continuously during about 50% of the whole blowing time. In that case, it is sufficient to replace about A: of the scrap by ore. Furthermore, it was found that for this purpose, an ore with a higher silicate content of about 5% which normally is not suitable for being added in the converter, is particularly advantageous. The ore can be added in the form of pellets or as fine lumps.

The use of ore in place of scrap permits a higher hot metal input per ton of steel. It has been found that to get the same effect without the use of ore, fluxes such as fluorspar, bauxite, or feldspar should be added to the lime being blown into the melt. It has been found that when used in lump form these have relatively little effect, but when used in powder form they are very effective. The formation of coarse dust was avoided by the addition of bauxite to the lime blown in at the beginning of the refining process.

The effectiveness of the process according to the invention is improved by tuyeres having an inclination of about 10 against the longitudinal axis of the convertor in such sense that the bath is given a rotational movement around this said axis.

The process according to the invention permits refining of low-phosphorus pig iron of a composition as normally used for the oxygen top blowing process, in a particularly economic manner, but it offers also the advantage that within wide limits there exists no dependence of the metallurgical behavior on the composition of the pig iron. Moreover, it was found that applying the teaching according to the invention, FeO contents in the slag of about 15% are attainable, resulting in a considerable increase in yield compared to the known oxygen top blowing processes. As described in our earlier filed application Ser. No. 821,802 of which the present application is a continuation-in-part, the process according to the invention is preferably conducted in a bottom blown convertor, which, in accordance with another feature of the invention comprises a steel jacket with a refractory lining and an inserted refractory bottom containing nozzles, at least one nozzle consisting of a feed pipe for oxygen together or without lime powder or other solids surrounded by a concentric feed pipe for the jacket fluid.

A particularly quiet refreshing process, involving very little splashing out, is obtained by using the following operational quantities. Oxygen pressure at the tuyeres is between 3 and 20 atmospheres gauge. The oxygen contains between 1 and 2 kg/Nm of powdered lime in average of total flow. The pipe conveying the reagents has an internal diameter of about 1/ 25th of the depth of the melt. The velocity of flow of the oxygen containing the powdered lime is about 30% less than that of the oxygen containing no lime. For example using a blow pressure of 8 atmospheres gauge, the pure oxygen flows at 300 Nm /h.cm of nozzle cross section. For example in the case of a convertor with a capacity of 30 tons, the depth of the melt being 0.6-0.7m, there were consumed 50 Nrn of oxygen per ton of steel produced. The blowing period was required to be at most 12 minutes. The rate of flow of oxygen was therefore 7,500 Nm /h. On this basis the necessary total nozzle cross sectional area is calculated to be 7,500/200 38 cm approximately. The depth of the melt being 0.6-0.7m, the best suitable nozzle diameter is 2.50m. There are therefore required 38/1.25 (3.14) 8 nozzles.

A further example of the oxygen consumption of a 200 ton vessel amounts to approximately 10,000 Nm /heat. Oxygen pressure is between 3 and 20 atmospheres gauge and in this example the oxygen pressure is approximately 10 atmospheres.

The whole blowing period is usually between 8 and 12 minutes; for this example it is assumed to be approximately 10 minutes. The oxygen flow rate is therefore approximately 60,000 Nm /h. The total blowing section for the oxygen then amounts to 60,000 Nm hz2 50 Nm /(h. cm =240 cm approximately. With a bath depth in the range of 1 to 1.5 m, (e.g. 1.2m), a ratio of 1D. of the oxygen tube to bath-depth of 1/25 results in an inner diameter of the oxygen tube of 48 mm. The diameter of 48mm corresponds to a circular section of approximately 18 cm Therefore 13 tuyeres with an oxygen tube of 48 mm ID. have to be installed.

This calculation gives the inner diameter of the oxygen tube for an optimal blowing behavior of the melt.

An inclination of the tuyeres of with respect to the longitudinal axis of the convertor, as described in application Ser. No. 101,637 was assumed in the calculation. A similar blowing behavior can be achieved within a range of roughly i 25% of the inner diameter of the oxygen tuyeres calculated above.

A lower oxygen pressure allows a larger ID. of the oxygen tuyeres, for example, with an oxygen pressure of 3 atm, the ID. of the oxygen tuyeres can be enlarged by approximately 30%. A higher oxygen pressure leads to a reduction of the [.D. calculated above by approximately for an oxygen pressure of atm.

The suspension of reagent is conveyed into the convertor, for example, through a conical pot mounted under the convertor floor, the mixture reaching the pot through a tangential supply pipe and leaving the pot through at least one distributor nozzle feed pipe leading away radially away from the pot and situated at a distance from the supply pipe. A very even distribution of the powdered lime is obtained by connecting the supply pipe to the distributor pot near the top, and connecting several equally spaced distributor nozzle feed pipes to the lower part of the pot.

Alternatively the convertor may have a cylindrical distributor pot mounted underneath the convertor floor, the cylindrical distributor pot being sub-divided into two compartments by a horizontal intermediate partition wall which is permeable to gas. The suspension is fed into the upper part of the pot through a reagent supply pipe. Pure oxygen is fed to the lower part of the distributor pot through an oxygen supply pipe. At least one distributor feed pipe is connected radially to the upper part of the pot above the reagent supply pipe. Preferably however the suspension of reagents is distributed to the convertor nozzles through several reagent distributor pipes spaced at equal distances apart and connected radially to the upper part of the distributor pot above the reagent supply pipe.

The blowing gas issuing from each nozzle in the bottom blown convertor of this process is preferably pure oxygen in a central core surrounded by a hydrocarbon containing gas or liquid such as propane, butane, natural gas, methane, coke oven gas, fuel oil, cerosin, benzine, alcohol, suspension of liquid hydrocarbons and water.

For the jacket fluid hydrocarbons have been found to be particularly suitable. These jacket fluids have a cool ing effect around the nozzle outlets, and effectively protect the nozzles and also the convertor bottom from wear. In practical operation, a particularly high reliability of the tuyeres has been proven in the use of a protective fluid containing hydrocarbon in an effective amount up to about 5.8% by weight of the oxygen. The quantity of the protective media is the average over the refining period. The oxygen conveying the powdered lime in suspension can if desired be blown into the melt at an angle with respect to the surface of the melt, so as to produce a circulation in the melt. For this purpose the nozzles themselves are inclined at an angle.

The powdered lime can if desired be blown into the convertor through all the nozzles. Alternatively some of the nozzles can be fed with pure oxygen, in which case these pure oxygen jets are also protected with sheaths of jacket fluid.

A particularly efficient utilization of the slag forming substances, mainly the lime, is obtained by blowing them equally distributed through all the nozzles. However the refining process may require a change in the quantities used during the refining.

The invention will be more readily understood with reference to three examples of preferred practices which are intended to illustrate the invention and not to limit it in any way.

EXAMPLE 1 60 tons of pig iron having contents of 4.2% carbon, 0.9% silicon, 0.8% manganese, 0.25% phosphorus, 0.050% sulphur, were charged into a bottom blown basic convertor together with 113 tons of scrap and 4.5 tons of ore. A total amount of lime of 4.2 tons was injected as lime powder during the refining. The lime consumption was about 60 kg/ton of liquid steel. After the scrap and the pig iron were charged into the convertor, an amount of lime of about 600 kg was blown in together with nitrogen for about one minute, whereby the sulphur content was reduced from 0.050% to 0.012%. Then 250 Nm oxygen/min. together with a total of 1.5 tons of lime powder were injected during two minutes. Thereafter, refining was continued for another seven minutes using an average lime load of 0.7 kg/Nm of oxygen, and an amount of oxygen of 300 Nm lmin. Towards the end of the refining, 0.6 tons of lime powder together with an amount of oxygen of 250 Nm lmin. were added during one minute. The analysis of the heat at tapping showed contents of 0.02% carbon, 0.15% manganese, 0.008% phosphorus, and 0.015% sulphur. It is especially notable that due to the practice according to the invention, the nitrogen con tent was considerably reduced; it amounted to 0.0010%. The ore was added after the second, the fourth and the fifth minute, always in equal portions. During the whole refining period, propane was delivered as a protective medium to the annulus of the nozzles. With good success it was allowable to vary the volume ratio of propane to oxygen between 0.5:100 and 7: 100, but in the meantime it was preferred 3: 100. The propane pressure was between 4 and 6 atm.

EXAMPLE 2 Using the same composition of pig iron as in Example 1, the pig iron charge was 63 tons, the scrap charge 13 tons. No ore was added. During the first two blowing minutes, an amount of lime powder of 1.2 tons was added. After a total of 8 minutes blowing time, using an amount of oxygen of 350 Nm lmin. and a pulsating lime load of 4 kg/Nm of oxygen during a time of 10 sec., the remaining lime of 2.3 tons was injected within 3 minutes. The essential difference compared to Example 1 lies in the fact that the high remaining amount of lime was added at a carbon content of about 0.8%. The analysis of the finished steel showed contents of 0.50% carbon, 0.45% manganese, 0.015% phosphorus, 0.017% sulphur. From the above analysis can be seen that the process according to the invention permits to protect a considerable part of the manganese against slagging.

As protective medium, natural gas of the following analyses was used in this example: approximately CH,,, approximately 8% other components of general formular C H approximately 2%N The volume ratio of natural gas to oxygen varies between 2:100 and 18:100, preferably 7:100. The pressure of the natural gas was between 5 and 10 atm.

EXAMPLE 3 A pig iron of the same composition as in Example 1 was used for a heat, the charge of which consisted of pig iron and scrap. 58 tons of pig iron and 20 tons of scrap were charged, 3 tons of lime were added in form of the usual lump lime. During the first two minutes, one tone of lime powder was injected together with the oxygen; during the next seven minutes, an amount of oxygen of 300 Nm"/min. without the addition of lime was blown. During the last minute, 900 kg of lime pow der together with an amount of oxygen of 250 Nm /min. were added. The final analysis of that steel showed contents of 0.02% carbon, 0.17% manganese.

0.012% phosphorus, 0.014% sulphur, and 0.0013% nitrogen.

In this case fuel oil with a specific weight of approximately 0.9 kg/l was applied as a protective fluid. The oil addition varies between 4 l/min. and 35 l/min., normally an amount of 20 l/min. was preferred. The oil pressure was between 5 and atm. if every nozzle had an own supplying tube, or between to atm by using only one supply pipe and an especial distributor near the tuyeres.

What we claim is:

1. A process for refining low-phosphorus pig iron containing in percentage by weight 0.1-3% Mn, 0.1-2% Si, and between 0.02 and 0.5% P, in a convertor in which below the surface of a bath of said pig iron and in the refractory brickwork at least onecomposite tuyere is disposed comprising a central tube through which a stream of oxygen is introduced into said pig iron and an outer tube through which a liquid or gaseous protective agent is introduced into said pig iron sur- 8 rounding said oxygen stream which process comprises: introducing lime powder into said pig iron in said stream of oxygen, and in which process 30 to 60% of said lime powder is injected during the desiliconizing of said pig iron.

2. Process according to claim 1, in which the lime load of the stream of refining gas varies during the refining period.

3. Process according to claim 1 in which a part of the lime powder is introduced into the melt together with an inert gas and before the beginning of the oxygen refining.

4. Process according to claim 1 in which during a second refining phase, after removing most of the first slag, the blowing is carried out with up to 20% of the amount of lime powder used during the initial phase.

5. Process according to claim 1 in which lime powder is injected with high specific load, more than 3 kg/Nm during the last 10 to 20% of the refining period.

6. Process according to claim 1 in which the blowing is carried out with a pulsating lime load.

7. Process according to claim 1 including, in addition, the introduction of fine solids other than lime, together with the oxygen, said solids being selected from the group consisting of iron ore, fluorspar, bauxite, feldspar, soda ash, dolomite and magnesia.

8. Process according to claim 1 in which of the lime powder which is injected is of a grain size below 0.1 mm.

9. Process according to claim 1 in which the total amount of lime used in the refining is in the form of lime powder and is introduced together with oxygen into the melt below the surface of the molten bath. 

1. A PROCESS FOR REFINING LOW PHOSPHORUS PIG IRON CONTAINING IN PERCENTAGE BY WEIGHT 0.1-3% MN, 0.1-2% SI, AND BETWEEN 0.02 AND 0.5% P, IN A CONVERTOR IN WHICH BELOW THE SURFACE OF A BATH OF SAID PIG IRON AND IN THE REFRACTORY BRICKWORK AT LEAST ONE COMPOSITE TUYERE IS DISPOSED COMPRISING A CENTRAL TUBE THROUGH WHICH A STREAM OF OXYGEN IS INTRODUCED INTO SAID PIG IRON AND AN OUTER TUBE THROUGH WHICH A LIQUID OR GASEOUS PROTECTIVE AGENT IS INTRODUCED INTO SAID PIG IRON SURROUNDING SAID OXYGEN STREAM WHICH PROCESS COMPRISES: INTRODUCING LIME POWDER INTO SAID PIG IRON IN SAID STREAM OF OXYGEN, AND IN WHICH PROCESS 30 TO 60% OF SAID LIME POWDER IS INJECTED DURING THE DESILICONIZING OF SAID PIG IRON.
 2. Process according to claim 1, in which the lime load of the stream of refining gas varies during the refining period.
 3. Process according to claim 1 in which a part of the lime powder is introduced into the melt together with an inert gas and before the beginning of the oxygen refining.
 4. Process according to claim 1 in which during a second refining phase, after removing most of the first slag, the blowing is carried out with up to 20% of the amount of lime powder used during the initial phase.
 5. Process according to claim 1 in which lime powder is injected with high specific load, more than 3 kg/Nm3 during the last 10 to 20% of the refining period.
 6. Process according to claim 1 in which the blowing is carried out with a pulsating lime load.
 7. Process according to claim 1 including, in addition, the introduction of fine solids other than lime, together with the oxygen, said solids being selected from the group consisting of iron ore, fluorspar, bauxite, feldspar, soda ash, dolomite and magnesia.
 8. Process according to claim 1 in which 90% of the lime powder which is injected is of a grain size below 0.1 mm.
 9. Process according to claim 1 in which the total amount of lime used in the refining is in the form of lime powder and is introduced together with oxygen into the melt below the surface of the molten bath. 